The present invention relates to a power assisted steering system of a motor vehicle, having a steering gear for converting the driver's intended steering, based on his steering maneuver, into a displacement movement of the tie rods assigned to the steerable wheels and having a superimposing unit with which an additional steering angle in the same direction or in the opposite direction can be superimposed on the steering angle selected by the driver so that the gear ratio between the steering angle selected by the driver and the travel of the rod pivot point on the steering gear is variable with respect to the gear ratio selected by the steering gear. With regard to the technical background, reference is made to German patent document Nos. DE 100 32 340 A1 and DE 101 26 424 A1.
Steering, i.e., driving the steerable wheels of a motor vehicle, requires force, i.e., power. This is particularly true of the lower the rotational speed of the wheels being steered. The greatest power demand thus occurs in parking a vehicle with the wheels braked, i.e., when the steerable wheels are braked and are thus blocked to prevent them from rotating while at the same time being driven, i.e., steered. Furthermore, the power demand is especially high in the range of the two possible end positions, i.e., close to the maximum possible wheel angle, because, among other things, the lateral strain in the wheel tire increases with an increase in the wheel steering angle.
Steering systems and/or mechanical steering gears are usually designed so that the overall ratio becomes more direct in proximity to the end position, i.e., with rack-and-pinion steering in the area of the end positions of the rack travel, so the total steering angle applied by the driver of the vehicle to the steering wheel (steering maneuver) is limited to a reasonable level from one end position to the other. In addition, the tie rod forces increase greatly with an increase in steering angle owing to the change in gear ratio in the tie rods. Both result in an especially high power demand in the end positions of the rack travel (in the case of rack steering).
In conventional power assisted steering systems, the auxiliary power is applied mainly hydraulically. However, auxiliary power systems using electric motors are also known. Regardless of which auxiliary power system is installed, larger dimensions of the auxiliary power assist are necessary because of the great power demand of an auxiliary power steering system, especially in the end positions of the rack travel and/or in the range of large steering angles (near the respective end position). Thus, powerful motors or hydraulic pumps must be provided, and their drive in turn has a high energy demand, so the burden goes to the energy balance, i.e., consumption by the drive unit of the motor vehicle, since this usually drives (directly or indirectly) the unit that supplies the auxiliary power for the steering system.
One possible remedy for this problem is described in German Patent DE 101 26 424 A1, according to which the steering ratio is designed to be more indirect in the end positions, so that the unit supplying the auxiliary power for the steering system (a so-called “servo device”) can be designed with the smallest possible dimensions. In the case of rack steering, a lower steering ratio in the end positions results in a lower rack travel per unit of steering angle selected by the driver by using the steering wheel or the like, so that logically a lower power is then required. However, as a direct result of such a steering ratio design, the steering angle range on the driver's steering wheel is increased when the steerable wheels are to be steered, e.g., from one end position to the other end position. To reduce this consequence which is uncomfortable for the driver while at the same time supplying the most indirect possible steering in the range of the zero position of the steering wheel, i.e., when driving straight ahead, according to DE 101 26 424 A1, the design of the steering gear is such that a relatively direct gear ratio is provided in an intermediate range between the zero position and the respective end position. The curve of the gear ratio, i.e., the rack travel (which is the travel of the tie rod pivot point on the steering gear) as a function of the steering angle selected by the driver with his steering wheel therefore has first an ascending area (starting from the zero position) and then has a descending area (at larger steering angles).
It has been found that a relatively direct steering ratio is still required in the range of the straight ahead position of the steering wheel, even with the known measure described here, if the total steering angle that must be applied to the steering wheel by the driver is to be kept within a reasonable size. One disadvantage of this is that in the range of the zero position, the steering should have a certain indirectness to be able to avoid extreme steering of the vehicle at small steering angles and thus ensure adequate stability. Another disadvantage of this known system according to German patent document No. DE 101 26 424 A1 is the fact that the gear ratio of the steering gear is represented fixedly (i.e., mechanically) in the rack contour, so it is impossible to implement different gear ratio curves.
In addition, steering systems with a so-called superpositioning unit are known. Such systems are capable of superimposing an extra steering angle acting either in the same direction or in the opposite direction upon the steering angle selected by the driver using the steering wheel (his steering maneuver in general). For example, this superimposing unit may be a servo motor having a planetary gear, whose first input is connected to the driver's steering wheel, whose second input is connected to a worm gear drive driven by the electric servo motor and whose output is mechanically connected to the input of the steering gear. Thus an additional steering angle can be adjusted via the electric motor with the worm gear drive and is then relayed to the steering gear.
For such a steering system the present invention provides a measure with the help of which the maximum power demand for driving the steerable vehicle wheels, which is especially high in the vicinity of both end positions, as already described above, and can be reduced without having to accept any great disadvantages.
The method of achieving this is characterized in that when a power limit for the system supplying the auxiliary power assist or a limit value for the steering angle specification is exceeded, at least when considered over time, the gear ratio between the steering angle selected by the driver and the travel of the rack pivot point on the steering gear is adjusted to be more indirect in comparison with the specified gear ratio.
In accordance with the present invention one of the measures to be described in greater detail below for reducing the power demand of the auxiliary power assist shall be implemented only as needed, i.e., essentially as a function of the power currently required from the unit supplying the auxiliary power assist. A comparison may be performed with a threshold value or a limit value (preferably specified), although that is not absolutely necessary. A measure for reducing the power demand may be implemented even if it is to be expected that a power limit is exceeded. However, if a concrete threshold value for the power demand is specified, it is preferably selected in such a way that it is reached only in extreme situations, e.g., in a parking operation with a very rapid steering to high friction value. Only if this threshold value is reached and/or exceeded is a measure for reducing the power demand initiated.
Initiating a measure for reducing the power demand on exceeding a limit value for the specified steering angle also goes in the same direction. Whenever the driver selects with his steering maneuver, in particular the steering wheel, a particularly large steering angle (or steering wheel angle) which is above a limit that has been set as suitable (and thus fundamentally requires a particularly high power for implementation), a measure for reducing the power demand is initiated.
In accordance with exemplary embodiments of the present invention a measure for reducing the power demand includes adjusting the gear ratio between the steering angle selected by the driver and the travel of the rack pivot point on the steering gear to be more indirect in comparison with a gear ratio specified by the steering gear, at least when considered over time. A more indirect gear ratio results in a reduced power demand. Then the displacement rate of the tie rod pivot point on the steering gear in comparison with the rate of change of the specified steering angle is reduced in the sense of a more indirect gear ratio by the fact that the aforementioned superimposing unit superimposes an additional steering angle in the opposite direction on the steering angle selected by the driver, i.e., the so-called superimposed steering adjusts a more indirect steering ratio after said limit value (power limit or steering angle limit) has been exceeded and thus in the case of rack steering, the rack traveling speed is lowered with no change in the intended steering angle speed. This avoids a need for maximum power, but it causes the required steering angle to be increased by the amount of the superimposed additional steering angle up to the end position of steering.
In the application of an inventive system, it is important to be sure that the driver steers back into the straight-ahead position along the same gear ratio curve in the return motion of his steering wheel (his steering maneuver in general) as in the original steering motion because otherwise the steering wheel is skewed at the height of the additional steering angle. Furthermore, the fact that the driver need not apply the same steering angle up to the steering stop position in all situations, depending on the power demand, so complete reproducibility is not obtained. Therefore, the maximum possible steering angle difference should not be perceived by the driver as causing interference. A design criterion for this function is thus a certain additional required steering angle demand.
Another possibility for reducing the power demand on the tie rods due to a lower travel speed of the rack of the steering gear or the like is by using a time-delay element in the controlled system. This function is very similar to the power-dependent steering ratio because at least when considered over time an indirect steering ratio is set. In accordance with the present invention the steering angle on the steerable vehicle wheels is delayed with respect to the steering movement or steering specification of the driver on reaching a certain power demand (i.e., the aforementioned power limit) or on exceeding a limit value for the specified steering angle (selected by the driver) and thus to build up the steering angle with a delay, i.e., on exceeding said limit value, the travel of the tie rod pivot point is readjusted with a time delay to the steering angle selected by the driver. The advantage of this function in comparison with the power-dependent steering ratio consists of the fact that the steering angle remains unchanged until reaching the (mechanical) steering stop. Depending on the time delay, the rack travel speed and thus the power demand on the tie rods may be reduced by a certain measure.
In accordance with exemplary embodiments of the present invention, the steering wheel angle and the steering angles on the steerable vehicle wheels are established independently of one another in principle, and the steering wheel is to reach the stop before the end of the steering operation on the wheel, so it may be advisable to provide an additional actuator or the like for displaying the steering stop in the steering column, for example, and/or with effect on the driver's steering wheel. This function may be taken over by an electric steering lock, for example.
As already explained, these measures should be initiated only on reaching or exceeding a limit value or threshold value for the steering wheel angle and/or the specified steering angle or for the power demand by the unit supplying the auxiliary power assist. Whether the limit value or threshold value is exceeded can be estimated from the respective profile of requirements or it is possible to ascertain in concrete terms that a power threshold has been reached. For example, to calculate the total power in the tie rods in the case of an electromagnetic servo steering (auxiliary power assist), the current consumed by the respective electric motor can be measured. In the case of a hydraulic steering assist, a suitable sensor system, e.g., in the form of wire strain gauges may be provided on the tie rods. If necessary, the current consumption by the superimposing unit which is driven by electric motor (superimposing steering) may also provide information regarding the power ratios in the tie rods and/or on the rack or the like of the steering gear.
In the sense of an estimate as to whether or not a power limit for the auxiliary power assist has been exceeded or will be exceeded, suitable measures to reduce the power demand may also be initiated in certain operating states. Such a measure will be taken when a reduction in power demand is in fact necessary, e.g., in a parking procedure. This can be described, for example, by the fact that the driving speed of the vehicle is less than 5 km/h. Furthermore, for example, steering when the vehicle is standing still and the wheels are braked and when the power demand is naturally high likewise represents an operating state in which a proposed change in the gear ratio is implemented, i.e., an additional steering angle in the opposite direction is superimposed in particular when the driver specifies a large steering angle; this is done in order to obtain a more indirect steering ratio.
Furthermore, in accordance with exemplary embodiments of the present invention the steering ratio is implemented essentially only in such a range in which an indirect setting will result in a relatively great reduction in power demand. Then in certain states or operating states at steering angles selected by the driver, which are above an intended steering angle that is essentially in the middle in the range between an intended steering angle of 0° and the maximum selectable steering angle, the travel of the tie rod pivot point on the steering gear which is executed at a preselected angle unit is reduced with an increase in the intended steering angle by superimposing of additional steering angles directed in the opposite direction. To reduce the steering demand on the steering gear, the steering ratio in the steering gear is designed to be more indirect but only in such areas and such operating states in which the power demand is or would otherwise be disproportionately high. In the area close to the end positions with the high power demand described above, the gear ratio is set to be more indirect by specifying a negative superimposed steering angle. The gear ratio curve thus has a falling section near the end positions and therefore reduces the power demand. This range with the “falling” section close to the end positions can also be described by the fact that it is above an essentially middle intended steering angle in the range between an intended steering angle of 0° and the maximum selectable steering angle.
In addition, the steering ratio may be set to be somewhat more direct in the area around the zero position (i.e., straight-ahead setting) in these certain operating states by adjusting positive superimposed steering angles by the superimposing unit because the measure described so far, namely to reduce the travel of the tie rod pivot point (on the steering gear) executed for each steering angle unit (preselected by the driver) by superimposing additional steering angles in the opposite direction (for relatively large steering angles in certain operating states and to thereby obtain a more indirect steering ratio), initially results in a larger total steering angle on the steering wheel from one stop to the next, but this is undesirable for comfort reasons, and this is preferably done in such a way that the required driver's steering angle remains essentially unchanged on the whole from one end position to the next.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.